The invention relates generally to combustors, and in particular to a trapped vortex combustor in a gas turbine.
In a conventional gas turbine engine, compressed air exiting from a compressor is mixed with fuel in a combustor. The mixture is combusted in the combustor to generate a high pressure, high temperature gas stream, referred to as a post combustion gas. The post combustion gas is expanded in a turbine (high pressure turbine), which converts thermal energy associated with the post combustion gas to mechanical energy that rotates a turbine shaft. The post combustion gas exits the high pressure turbine as an expanded combustion gas.
Some gas turbines deploy a reheat combustor to utilize the oxygen content in the expanded combustion gas. The expanded combustion gas is again combusted in the reheat combustor after adding additional fuel and the re-combusted expanded combustion gas is expanded in a second turbine (low pressure turbine) to generate additional power.
If the combustion process occurring in the combustor and the reheat combustor is incomplete/not efficient, the hot gases exiting from the combustor/reheat combustor will contain pollution causing elements such as partially combusted hydrocarbons, oxides of nitrogen etc. Such pollution causing elements are eventually discharged into the atmosphere after exiting from the high pressure turbine (or the low pressure turbine, if deployed). It is therefore necessary that the combustion process be efficient and complete.
Among the challenges to improve combustor efficiency include efficient mixing of fuel and air and stabilization of the resulting flame. One of the means for addressing these challenges is inclusion of a trapped vortex cavity located on the wall of the combustor. Fuel is injected into the trapped vortex cavity from certain fixed points within the cavity. A portion of the air entering the combustor (expanded combusted gas in case of a reheat combustor) is diverted towards the trapped vortex cavity, which as the name suggests, traps the portion of the air into forming a vortex. It is desirable to achieve a stable, high speed vortex, which helps in efficient mixing of the air with the fuel injected into the trapped vortex cavity. However, to achieve a stable vortex, air entering the combustor has to be accelerated to high speeds, which results in reduced gas turbine efficiency. Further, the injection of fuel from fixed points within the cavity often creates pockets of rich fuel in the vortex of air and does not achieve the desirable amount of mixing. An inefficient mixing and unstable vortex consequently results in an unstable flame, which in turn causes inefficient combustion.
It is desirable to create a stable vortex and achieve an efficient mixing of fuel and air in the trapped vortex cavity of the combustor.